systemd-networkd gets installed by default by Arch, integrates a bit better with the rest of SystemD, doesn't have so many VPN surprises, and the configuration is a bit more obvious to me - a few config files rather than NetworkManager's "loads of scripts" approach. Small niggles rather than big issues.

Really, I just don't want duplication of services - more stuff to keep up-to-date. And if I've got SystemD anyway, might as well use it...